In many varieties of switch mode power supplies, an output parameter—typically output voltage—is regulated by means of an error derived in an error amplifier on the output or secondary side of the SMPS; a signal corresponding to the error is transmitted to the primary or input side, typically by means of an opto-coupler to maintain electrical isolation between the input and output sides. Such power supplies are often used in combination with consumer applications, such as adapters for laptops, cell phones, TV, desktop PCs and the like.
Whereas opto-couplers used in such power supplies typically output a low current, for example of the order of 80 μA, this may still require a current supply to the LED of, say 400 μA for a current transfer ratio of 0.2. Under low-load, or “no-load” conditions, this current represents a significant fraction of the power consumed by the SMPS. Since the electronic equipment connected to many such power supplies is typically left in an “off” or “standby” state for extended periods of time, the corresponding low load or no-load power consumption of switch mode power supplies is coming under increasing scrutiny with tightening requirements on wasted power, and in particular the power dissipated under low load or no-load conditions.